Bi-mill for milling an opening through a wellbore casing and in a preplanned lateral drilling path in departure from the wellbore axis

ABSTRACT

A bi-mill mills an opening through a wellbore casing in a preplanned lateral drilling path in departure from the wellbore axis and through the wellbore casing or rock formation and in the direction of the drilling path. The bi-mill provides increase debris removal and cutting surfaces and includes an assembly having a lead mill and a follow mill and attaches to the wellbore whipstock. The lead mill further includes fluid passageways leading from the lead mill and exiting the nose of the lead mill. Debris removal channels channel milled debris away from the lead mill and in the direction of the follow mill form in a helical configuration for guiding debris milled by lead mill inserts along the debris removal channels. The lead mill and follow mill continue milling through the wellbore casing and into the underground rock formation a distance sufficient to initiate the preplanned drilling path.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional application 62/693,873 entitled “Bi-Mill For Milling An Opening Through A Wellbore Casing In Initiating With Helical Debris Channels And Dense Milling Inserts Configuration,” filed on Jul. 3, 2018 with Attorney Docket No. WILD003USP, which is here expressly incorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to oilfield drilling equipment and more particularly to equipment for milling through a wellbore casing, cement and rock formations in order to initiate lateral drilling operations. The disclosure relates more specifically to a bi-mill for milling an opening through a wellbore casing and in a preplanned lateral drilling path in departure from the wellbore axis using helical debris channels and dense milling insert configurations for initiating a lateral bore into a rock formation. Even more particularly, the present disclosure an improved lead mill having a novel shear bolt cavity and shear bolt retaining pin configuration, permitting a greater total flow area (“TFA”) for debris clearance and permitting the placement of extra cutting structure on the lead mill, particularly at the lead mill's nose and plateau area.

BACKGROUND OF THE DISCLOSURE

This disclosure applies in the field of wellbore departure milling. The novel features believed characteristic of the disclosed subject matter are set forth herein. The disclosed subject matter itself, as well as the preferred mode of use, further objectives, and advantages thereof, are best understood by reference to the following detailed description of an illustrative embodiment.

In modern drilling practices for oil and gas wells, it is common for a main wellbore to be drilled and for a sidetracking operation to be undertaken in order to drill a separate bore extending away from the main wellbore. Examples of such systems include the following:

U.S. Pat. No. 8,122,977, entitled “Cutting Device with Multiple Cutting

Structures” shows a cutting device for downhole operations that includes a first cutting structure, and a second cutting structure. In that disclosure, at least the second cutting structure is selectively presentable for operation. The disclosure includes a method of performing a downhole cutting operation that includes running into a well bore a cutting device including a plurality of cutting structures and performing a first cutting operation with a first cutting structure of the cutting device. Then, the disclosure shows moving the first cutting structure to selectively present a second cutting structure of the cutting device, and performing a second cutting operation with at least the second cutting structure. The method may further include aligning movable cutting structures of the cutting device to allow the second cutting structure to be selectively presented.

U.S. Pat. No. 8,459,357, entitled “Milling System and Method of Milling” shows a mill for milling a window through metal casing in a well bore that includes a body having a plurality of blades. In that disclosure, a plurality of cylindrically bodied cutting elements are position on the blades. A plurality of diamond enhanced elements include a non-planar diamond working surface that initiates cutting into the casing for milling a window into well bore.

Even more related, U.S. Pat. No. 8,997,895, entitled “System and Method for Coupling an impregnated Drill Bit to A Whipstock” shows a system and method that facilitate the drilling of one or more lateral wellbores while eliminating one or more trips downhole. The system uses a drilling assembly including an impregnated drill bit or other suitable drill bit. The impregnated drill bit couples to a whipstock by a connector for deployment downhole in a single trip. The connector includes a separation device which facilitates disconnection of the impregnated drill bit from the whipstock once the whipstock is anchored at a desired downhole location.

While the above and similar patents represented improvements at their time, today a need has arisen for a lead mill that makes efficient use of space to permit a greater total flow area (“TFA”) for debris clearance than in the prior art. Such an improved configuration also has the demand for providing extra cutting structures to be placed on the lead mill compared to prior art designs, particularly at lead mill's nose and plateau area. If such designs could be achieved, then the lead mill and follow mill could more efficiently and rapidly continue milling through the wellbore casing or wellbore rock formation wall and into the rock formation a distance sufficient to initiate the preplanned drilling path. These improvements will enhance the overall profitability of energy production projects, reduce energy production costs, and provide greater energy resources to the consuming public in general.

BRIEF SUMMARY OF THE DISCLOSURE

The present disclosure details a method, system, and fabrication method for a bi-mill for milling an opening through a wellbore casing and in a preplanned lateral drilling path in departure from the wellbore axis using helical debris channels and dense milling insert configurations for initiating a lateral bore into a rock formation.

According to one aspect of the present disclosure, there is here provided a bi-mill for milling an opening through a wellbore casing in initiating a preplanned lateral drilling path in departure from the wellbore axis and through the wellbore casing or wellbore rock formation, and further continuing into underground rock formation outside the wellbore and in the direction of the preplanned lateral drilling path and as guided by a wellbore whipstock.

The bi-mil includes an assembly having a lead mill and a follow mill. The lead mill threadably attaches to the follow mill. The lead mill includes a body forming a structural base for the lead mill, and further includes a smooth bore in the lead mill into which a circumferentially-grooved shear bolt is inserted. A threaded end threadably attaches to the wellbore whipstock and with the unthreaded end inserts into the smooth bore. The lead mill further includes a plurality of fluid passageways wherein the smooth bore has a sufficiently shallow depth so as to not intersect with the internal fluid passageways and so as to not exit the distal side of the lead mill. The lead mill further includes an external bore and a retaining pin inserted through the external bore for intersecting the smooth bore.

The shear bolt includes a groove, wherein the retaining pin further inserts into the groove for retaining the shear bolt within the smooth bore. The lead mill further includes a nose and at least four fluid passageways leading from a central axial fluid passageway inside the lead mill and exiting a plateau defined as the end portion of the nose with an area perpendicular and obtuse in relation a central fluid passageway of the lead mill. A plurality of debris removal channels form recessed paths within the body and a plurality of exit paths for channeling milled debris away from the lead mill and in the direction of the follow mill. A plurality of milling inserts is positioned on the lead mill and in rows adjacent the debris removal channels for milling through the wellbore casing or wellbore rock formation and into the underground rock formation. At least three rows of milling inserts extend from the center of the plateau down the side of the lead mill in continuity.

The debris removal channels further form in a helical configuration for guiding debris milled by the inserts along the debris removal channels and away from the lead mill and in the direction of the follow mill. The lead mill and follow mill are configured to follow a path defined by the wellbore whipstock to continue milling through the wellbore casing and into the underground rock formation a distance sufficient to initiate the preplanned drilling path.

In another aspect of the present disclosure, here are disclosed methods, devices, and systems provide a bi-mill for milling an opening, guided by a wellbore whipstock, through a main wellbore casing, or wellbore rock formation wall, in order to initiate a preplanned subsequent drilling path in departure from the wellbore axis and through the wellbore casing, and further continuing into underground rock formation outside the main wellbore and in the direction of the preplanned lateral drilling path. The bi-mill includes a lead mill and a follow mill assembly, the lead mill attaches to the follow mill, and both are controlled by rotation of a work string or drill string. The lead mill includes a body forming a structural base for the lead mill. The follow mill also includes a body forming a structural base for the follow mill. The lead mill body includes a plurality of debris removal channels formed as recessed paths within the body and forming a plurality of exit paths for channeling milled debris away from the lead mill and in the direction of the follow mill.

A plurality of milling inserts are positioned on the lead mill and positioned in arcing rows of body material adjacent to the debris removal channels, creating a cutting structure for milling through the wellbore casing or rock formation wall and into the adjacent underground rock formation. A central, axial bore in the bi-mill carries fluid pumped from the surface during the milling operation, with the fluid exiting at a plurality of orifices located at the plateau of the lead mill's nose. The debris removal channels are further formed in a helical configuration for guiding debris milled by the inserts along the debris removal channels and away from the lead mill and in the direction of the follow mill.

The follow mill body has threaded ends, and does not therefore possess a “nose” such as the end of the lead mill. However, the follow mill also includes a plurality of debris removal channels formed adjacent to an arcing cutting structure as recessed paths within the body and forming a plurality of exit paths for channeling milled debris away from the follow mill and in an uphole direction. A plurality of milling inserts are positioned on the follow mill and positioned in rows adjacent to the debris removal channels, creating a cutting structure for milling through the wellbore casing or rock formation wall and into the underground rock formation. The debris removal channels are further formed in a helical configuration for guiding debris milled by the inserts along the debris removal channels and away from the follow mill and in an uphole direction.

The lead mill makes efficient use of space with a novel shear bolt cavity and shear bolt retaining pin configuration, permitting a greater total flow area (“TFA”) for debris clearance than in the prior art. This space-saving configuration also permits extra cutting structure to be placed on the lead mill compared to prior art designs, particularly at lead mill's nose and plateau area.

Still further objects, technical aspects and advantages of the presently disclosed lead mill and follow mill configured to follow a path defined by the whipstock to continue milling through the wellbore casing or wellbore rock formation wall and into the rock formation a distance sufficient to initiate the preplanned drilling path will become apparent upon reading the technical description and considering the claims appearing below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present subject matter will now be described in detail with reference to the drawings, which are provided as illustrative examples of the subject matter so as to enable those skilled in the art to practice the subject matter. Notably, the FIGUREs and examples are not meant to limit the scope of the present subject matter to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements and, further, wherein:

FIG. 1 shows a typical workstring configuration for wellbore departure milling;

FIG. 2 shows a bi-mill comprised of a lead mill, a threaded tubular connection, and cylindrical inserts on both the lead mill and follow mill;

FIG. 3 shows the lead mill component of the bi-mill in the state of being attached to the upper portion of a whipstock;

FIG. 4 shows a more comprehensive isometric view of the bi-mill and adjoining whipstock;

FIG. 5 presents a section view of the lead mill a portion of the whipstock with the lead mill shown attached to the whipstock with a shear bolt;

FIG. 6A shows an isometric view of the bi-mill with follow mill, lead mill, and some capped shearable nozzles at the nose of the lead mill;

FIG. 6B shows a section view of the bi-mill, with the shear bolt cavity visible as well as capped shearable nozzles;

FIG. 7A shows an isometric view of the follow mill, lead mill, and the threaded connection that joins them together when assembled;

FIG. 7B shows a section view of the follow mill, lead mill, and threaded connection that joins them together when assembled;

FIG. 8A shows the lead mill in isometric representation, with capped shearable nozzles visible at the nose of lead mill;

FIG. 8B shows the lead mill in isometric representation, with capped shearable nozzles visible at the nose of lead mill;

FIG. 9 shows an isometric view of the lead mill with nozzle ports visible at the plateau of the nose of the lead mill;

FIG. 10 shows an enlarged view of a portion of lead mill, with a threaded break-off nozzle at the plateau of the nose of the lead mill; and

FIGS. 11A and 11B are graphical representations of the overlap of all the cylindrical inserts shown as if arranged in a single plane.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments in which the presently disclosed subject matter can be practiced. The term “exemplary” used throughout this description means “serving as an example, instance, or illustration,” and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for providing a thorough understanding of the presently disclosed method and system. However, it will be apparent to those skilled in the art that the presently disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and devices are shown in functional or conceptual diagram form in order to avoid obscuring the concepts of the presently disclosed method and system.

In the present specification, an embodiment showing a singular component should not be considered limiting. Rather, the subject matter preferably encompasses other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, the applicant does not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present subject matter encompasses present and future known equivalents to the known components referred to herein by way of illustration.

One or more embodiments of the disclosure are described below. It should be noted that these and any other embodiments are exemplary and are intended to be illustrative of the disclosure rather than limiting. While the disclosure is widely applicable to different types of systems, it is impossible to include all the possible embodiments and contexts of the disclosure in this disclosure. Upon reading this disclosure, many alternative embodiments of the present disclosure will be apparent to the person's ordinary skill in the art.

The benefits and advantages that may be provided by the present disclosure has been described above with regard to specific embodiments. These benefits and advantages, and any elements or limitations that may cause them to occur or to become more pronounced are not to be construed as critical, required, or essential features of any of any or all of the claims. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It is further understood that the terms “comprises” and/or “comprising” or “includes” and/or including”, or any other variation thereof, are intended to be interpreted as nonexclusively including the elements or limitations which follow those terms. Accordingly, a system, method, or other embodiment that comprises a set of elements is not limited to only those elements, and may include other elements not expressly listed or inherent to the claimed embodiment. These terms when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more features, regions, integers, steps, operations, elements, components, and/or groups thereof.

Although the present disclosure provides a description of a bi-mill having a lead mill and follow mill configured to follow a path defined by the whipstock to continue milling through the wellbore casing or wellbore rock formation wall and into the rock formation a distance sufficient to initiate the preplanned drilling path, it should be understood that the description is by way of example only and is not to be construed in a limiting sense. It is contemplated that all such changes and additional embodiments are within the spirit and true scope of this disclosure, as claimed below.

This disclosure facilitates the exit from the casing, or rock formation, in the absence of casing, and creation of an initial bore over a short distance into the rock formation. A sidetracking or lateral drilling operation begins with what may be referred to as a “wellbore departure” operation. The objective in wellbore departure is to exit from a main wellbore and begin boring a new lateral bore that exits or departs from the main wellbore.

In a main wellbore that is either “open hole,” i.e., without casing cemented in the wellbore, or “cased hole,” i.e., with casing cemented in the wellbore, wellbore departure requires an assembly that includes a few key components. An anchoring system on a workstring must be deployed below, i.e. farther downhole, from a whipstock on the same workstring. The anchor may be of a mechanical or hydraulic type, both of which have many varying embodiments that are known in the art, including packers, plunger-actuated mechanical anchors, and hydraulic anchors that operate with supplied fluid pressure. The anchor must be “set,” which is to say it must be actuated so that it fixedly remains in place, in order to support the whipstock above it. When the anchor is set, the whipstock is also fixed in position in the wellbore so that it is impervious to compressive force and torque from a connected workstring. The hydraulic anchor used in this disclosure sets, or engages the wellbore so as to fixedly lock in place, when a bypass valve located uphole in the workstring receives sufficient flow and pressure to actuate, passing flow and pressure to a piston inside a running tool downhole from the bypass valve but uphole and in close proximity to the bi-mill. The piston inside the running tool is actuated, sending a pressurized clean fluid through the bi-mill, through the lead mill. Flow continues through a tubular item attached to the lead mill continuing through a tubular item in the whipstock and to a piston inside the anchor, actuating the piston. When the piston in the anchor is actuated, it drives slips or dogs into the wellbore wall and an internal locking mechanism holds them in place.

After the anchor is set in place, noting that the anchor shown in this disclosure is a hydraulic anchor but could conceivably be any style of anchor, a milling operation can begin. The mill(s) used for wellbore departure milling may be comprised of various components and configurations known in the art, usually an arrangement of two or three mills connected in close proximity. The milling system in this disclosure is a bi-mill, that is to say, a two-mill system. This bi-mill is comprised of a lead mill that is threadably connected to a follow mill, with the lead mill being positioned farther downhole.

In this disclosure, the bi-mill is attachably connected to the whipstock with a shearable fastener known as a “break bolt” or “shear bolt.” Attaching the bi-mill to the whipstock with a shearable item enables the whipstock-setting and wellbore departure milling operation to be completed in a single trip into the wellbore. After the anchor has been set in place as described above, and thereby the connected whipstock has been set in place, force, such as tension or compression from the workstring, is used to shear the shear bolt so that the connection between the whipstock and bi-mill is severed. Incidentally, the hydraulic tubular, an expendable item, connected to the mill, is destroyed at this stage or in the following initiation of milling. At this point, rotation of the bi-mill begins, and downhole compressive force is applied, forcing the bi-mill down the whipstock ramp and against the wellbore's internal casing wall or rock formation, as the case may be. In a cased well, the bi-mill creates an opening in the wellbore casing and a bore through any cement and into rock formation. When no casing is present, the bi-mill begins immediately boring into rock formation. The opening and new adjoining bore, separate from the main wellbore, are enlarged to a desired maximum diameter, or enlarged to “full gage,” in common oilfield terminology. The bi-mill bores a short distance in the new adjoining bore, forming a “rathole” in the rock formation in which the lateral bore will be drilled, and then ceases its operation and is withdrawn from the well. A drilling system designed for boring in rock formations is then tripped into the new adjoining bore in order to extend that bore a comparatively great distance.

A bi-mill assembly has a lead mill, which mills through the casing or rock formation wall ahead of the adjoining follow mill. Although follow mills may be of different types and forms, and may be relied upon to perform a large or small part of the milling operation in terms of material removed, the lead mill in this disclosure performs the bulk of the milling, leaving mostly smoothing or “cleanup” work for the follow mill in most situations.

However, the lead mill may experience some wear and breakage as it leads the milling operation, contacting, milling and penetrating the casing ahead of the follow mill. Significant wear on the lead mill, should it occur, can cause the lead mill's diameter to decrease from the desired full gage. The follow mill, which has been engaged in relatively light duty compared to the lead mill, helps to ensure that full gage is achieved in the main wellbore opening and in the adjoining bore created beyond the main wellbore. After creating the opening in the main wellbore and departing into the new, adjoining bore, the bi-mill bores a short distance in rock formation before being withdrawn from the well.

Referring now to FIG. 1 which shows a typical workstring configuration for wellbore departure milling, including the bi-mill 8, whipstock 14, and hydraulic anchor 16, with the hydraulic anchor 16 located distal and downhole from the bi-mill 8, as well as ancillary components. The bypass valve 2 located uphole in the workstring receives sufficient flow and pressure to actuate, passing along flow and pressure to a piston (not shown) inside a running tool 6 downhole from the bypass valve 2 but uphole and in close proximity to the bi-mill 8. The piston inside the running tool 6 is actuated, sending a pressurized clean fluid through the bi-mill 8, to and through the lead mill 12, through a hydraulic tubular 18 item attached to the lead mill 12 continuing through a tubular item (not shown) in the back of the whipstock 14, through a hinged connector 20 and into the hydraulic anchor 16 to pistons (not shown) inside the hydraulic anchor 16, actuating the pistons sequentially. When the pistons in the anchor are actuated, they drive slips 22 into the wellbore wall and an internal locking mechanism (not shown) within hydraulic anchor 16 holds the slips 22 in place.

FIG. 2 shows a bi-mill 8 comprised of a lead mill 12 at the lower end connected, by a threaded tubular connection 20, to a follow mill 10 at the upper end. Also seen in FIG. 2 are cylindrical inserts 30 on both the lead mill 12 and follow mill 10. These cylindrical inserts 30 are comprised of a hard material, such as tungsten carbide with a high percentage (e.g. 10.5% -12%) of cobalt or may be comprised of carbide with a top layer of polycrystalline diamond (“PCD”) on the exposed cutting face. Such inserts are designed to be wear and break resistant as well as useable with significant heat. On lead mill 10, wear insert holes 33 are shown at the rear portion of the cutting structure, but no wear inserts are shown inserted in these holes. Wear inserts are an optional item that may be utilized for protecting the mill body and preventing wear.

FIGS. 3, 4 and 5 show the lead mill 12 component of the bi-mill in the state of being attached to the upper portion of a whipstock 14. FIG. 3 shows an isometric view of a portion of the lead mill 12 and whipstock 14, as these parts are connected and in the assembled position for tripping downhole. A hydraulic tubular 18 conveys fluid from the running tool (not shown in FIG. 3) that passes through the lead mill to a hydraulic anchor (not shown in FIG. 3). One of the nozzle ports 34 is shown at the plateau of the nose of the lead mill 12. FIG. 4 shows a more comprehensive isometric view of the bi-mill 8 and adjoining whipstock 14. In FIG. 5, a section view of the lead mill 12 and section view of a portion of the whipstock 14 are seen, with the lead mill 12 shown attached to the whipstock 14 with a shear bolt 24. FIG. 5 also shows the shear bolt 24 being threaded into the whipstock 14 and retained in the lead mill 12 inside a shear bolt cavity 23 by a recessed threaded retaining pin 26 that interferes with a circumferential groove in the shear bolt 24. A hydraulic tubular 18, used for actuating the hydraulic anchor 16 (shown in FIG. 1), is threaded into a port at the nose or “plateau” of the lead mill. Capped, shearable nozzle 28 assemblies are also threaded into the plateau of the lead mill.

FIG. 6A shows an isometric view of the bi-mill 8 with follow mill 10, lead mill 12 and some capped shearable nozzles 28 at the plateau of the nose of lead mill 12. FIG. 6B shows a section view of the bi-mill 8, with the shear bolt cavity 23 visible as well as capped shearable nozzles 28.

FIG. 7A shows an isometric view of the follow mill 10 and lead mill 12 and the threaded connection 20 that joins them together when assembled. FIG. 7B shows a section view of the follow mill 10 and lead mill 12 and the threaded connection 20 that joins them together when assembled.

FIG. 8A shows the lead mill 12 in isometric representation, with capped shearable nozzles 28 visible at the plateau of the nose of lead mill 12, the shear bolt cavity 23, and cylindrical inserts 30 visible. FIG. 8B shows the lead mill 12 in isometric representation, with capped shearable nozzles 28 visible at the plateau of the nose of lead mill 12, the shear bolt cavity 23, and cylindrical inserts 30 visible.

FIG. 9 shows an isometric view of the lead mill 12 with nozzle ports 34 visible at the plateau of the nose of the lead mill 12. Nozzles have not been threaded into the nozzle ports 34 in this view. Notable in nozzle ports 34 are the smooth circumferential spaces in the inside diameter just above and adjacent to the threaded area in the inside diameter. These smooth recesses are designed to receive the hex heads of threaded items such as capped shearable nozzles 28. This allows the hex head to be recessed below the surface, with only the grooved break-off portion of a capped shearable nozzle 28 exposed even with the surface. The end result of this is a flush-with-surface breaking off of capped shearable nozzles 28 or other shearable items. By breaking off flush with the lead mill 12 debris removal channels 38, better flow paths and better, more efficient removal of debris is enabled. The increase in flow path and debris removal efficiency leads to less wear and longer cutting life of lead mill 12.

FIG. 10 shows an enlarged view of a portion of lead mill 12, with a threaded break-off nozzle 37 at the plateau of the nose of the lead mill 12. The lower grooved portion of threaded break-off nozzle 37 illustrates the low breaking off point that permits better debris removal and longer lead mill 12 life as described in previous FIG. 9. Note that the hex head of this nozzle is not visible, as it is submerged below the surface of the debris removal channel. Again, breaking off flush with the upper edge of nozzle port 34 permits the enhanced debris removal and longer lead mill life as described in FIG. 9.

FIGS. 11A and 11B are graphical representations of the overlap of all the cylindrical inserts 30 shown as if arranged in a single plane. This graphical representations indicate that strategic design has left no gaps in the spacing of cylindrical inserts 30, or cutting structure, and that many inserts share similar cutting loads, reducing torque and increasing insert and mill life. Little breakage occurs with such complete insert coverage and rate of progress (“ROP”) is markedly increased.

The disclosed invention is an apparatus for milling an opening in wellbore casing or wellbore rock formation and creating a bore beyond the casing that is discrete from the main wellbore. The subject matter of this disclosure is a bi-mill 8. The bi-mill 8 is comprised of a lead mill 12 with a single follow mill 10. Each of these mills, lead mill 12 and follow mill 10, is comprised of a body with an arcing “cutting structure” in relief from debris removal channels 38 that includes an arrangement of rows of cylindrical inserts 30 of a hard material brazed into pockets in the body, and recessed debris removal channels 38 formed between the rows of inserts. The lead mill 12 has a body with recessed debris removal channels 38 and fluid delivering orifices, potentially including three nozzle ports and a tubular connection port for flushing debris, with these ports being located on the plateau, or nose, of the lead mill. When used in conjunction with a hydraulic anchor, the nozzle ports are capped with capped shearable nozzles 28 so that fluid will not escape through these ports and may be supplied under pressure through one port connected to a hydraulic tubular 18 for hydraulic anchor 16 actuation.

The nozzle ports 34 are designed with threaded connections recessed below the surface of the mill body and shear points or circumferential grooves positioned at or below the surface of the mill body so that the nozzles break off flush with the surface of the adjacent area. A smoothly bored area within the upper inside diameter of the nozzle ports 34 is sized to accept the hex head portion of a threaded nozzle. This smoothly bored area may exceed the diameter of the threaded area of the nozzle port 34 if necessary. The smoothly bored portion of nozzle port 34 permits the hex head portion of the threaded nozzle to be fully submerged below the surface of the debris removal channel 38 or other surface area of lead mill 12. This leaves the shear groove of the capped shearable nozzle 28 or threaded break-off nozzle 37 in position to break off flush with the upper edge of nozzle port 34. Flush breaking off of the nozzles permits the enhanced debris removal, smoother cutting and longer lead mill life as detailed in FIG. 9 and FIG. 10.

Placement of a recessed shear bolt cavity 24 that is bored to a shallow depth in the lead mill body, i.e. not bored to the central axial fluid carrying bore within the lead mill 12 or bored through to exit the other side of the lead mill, and held by a threaded retaining pin 26, preserves sufficient internal space at the plateau of the lead mill 12. This is sufficient to enable efficient geometric placement of three nozzles and a tubular connection, or alternatively, two large diameter nozzles and a tubular connection, or four nozzles at the plateau of the lead mill 12. Regardless of the number of orifices at the plateau, these orifices can be of larger diameter than in the prior art due to the novel use of the lead mill's internal space. The ability to place more nozzles at the plateau, and to increase total flow area (“TFA”) at the plateau, is critical in achieving efficient debris removal, reducing required torque and reducing potential for slip-stick. Slip-stick occurs when a mill or bit binds during workstring rotation, stores energy until it releases, and then releases with unwanted rotational acceleration, causing vibratory and potentially destructive effects.

In this embodiment, cylindrical inserts 30 with a diameter of 13.437 mm are used, with the inserts being composed of a sintered carbide with a polycrystalline diamond (“PCD”) layer on top of the cylinder at its cutting face, or, alternatively, utilizing no PCD layer but simply tungsten carbide containing cobalt of 10.5% for improved breakage resistance and longer wear. The portion of the milling operation in which the bi-mill contacts steel casing involves significant vibration and interrupted, inconsistent contact with the casing. Such conditions necessitate employing a breakage-resistant carbide that must be somewhat softer, i.e., contain more cobalt, than a normal machining grade of carbide insert.

The lead mill's recessed debris removal channels 38 benefit from three shearable nozzles 28 and a tubular connection hole 32 at the plateau of the lead mill 12. Three shearable nozzles 28 with nozzle ports 34, in addition to a tubular connection hole 32 located at the plateau provide for significantly increased total flow area (“TFA”) at the plateau when compared to the prior art. The increased TFA provides for maximal upflow of milled debris, and a resultant reduction in required torque, producing a smoother milling operation. The greater TFA and torque reduction provides less opportunity for vibration destructive slip-stick. Slip-stick occurs when a mill or bit binds during workstring rotation, stores energy until it releases, and then releases with unwanted rotational acceleration, causing vibratory and potentially destructive effects.

The lead mill 12 possesses a novel geometry that makes efficient use of space and enables greater TFA at the nose of the mill. The lead mill 12 employs a recessed, blind cavity, not through-bored to opposite side of the lead mill as in prior art, nor bored to the fluid-carrying axial bore inside the lead mill, but rather with shear bolt cavity 23 bored transverse to the lead mill's axial plane, and with the shear bolt 24 is held in place by retaining pin 26. The internal space saved by utilizing a short, shallow, blind bore in the shear bolt cavity 23 leaves enough internal space to bore three fluid nozzle ports 34 and a tubular connection hole 32 to exit on the plateau of the nose of the lead mill 12. Furthermore, the fluid-delivering orifices, in this embodiment fluid nozzle ports 34 and tubular connection hole 32 are of larger diameter than similar prior art orifices. This use of space both concentrates and increases flow at the plateau. All exterior connecting parts, such as shearable nozzles 28 or hydraulic tubular 18, are fastened with recessed bolt heads and with shear points flush with the adjoining area of the lead mill 12. These parts break off flush when the milling operation begins, leaving no obstruction to milling or debris flow paths, and thereby providing maximum flow efficiency for removal of milled debris.

The lead mill 12 body, when machined, leaves substantial insert cutting structure after creation of the debris removal channels 38. Improving upon prior art cutting structure, the lead mill 12 body has three complete rows of inserts from the center plateau of the nose of the lead mill 12 and extending completely down along the side of the lead mill 12. The extra insert cutting structure at the plateau is achieved by recessing the hex heads and shear points of all shearable nozzles 28 or hydraulic tubular 18 parts, as described above in this disclosure. Recessing these hex heads allows more body to remain in place, i.e. leaving more body in relief as debris removal channels 38 are milled in manufacturing, providing sufficient strength to increase body area that may be utilized as insert cutting structure. The lead mill 12 body has six complete rows of inserts 30 from the nose (including the three rows from plateau, above) and extending completely down the side of the lead mill 12. Three additional segments of the lead mill 12 body along the side of the lead mill 12 offer additional insert cutting structure. Altogether, the lead mill 12 body in this embodiment contains nine rows of inserts 30.

The lead mill and follow mill inserts 30 are made of a hard material designed to resist wear and breakage, while still maintaining the ability to mill through steel and bore at least a short distance in rock formation to create a “rat hole” for accepting a rock drilling assembly following the milling operation. The hard insert material may be a polycrystalline diamond (PCD) sintered as a layer on top of a given type of carbide, or what is commonly known as a polycrystalline diamond compact (PCD) insert. Alternatively, a type of sintered carbide, such as tungsten carbide or a tungsten carbide insert (TCI) material containing cobalt content of at least 10.5% may be utilized. Additionally, PCD inserts may be used in conjunction with carbide inserts, such as TCI inserts, in a varied array of different material inserts placed into the lead mill 12.

When executing a milling operation to create an opening in casing, the lead mill's nose, the narrower portion of the lead mill 12 that is less than “full gage,” travels along a whipstock 14, initially contacting the whipstock 14 as the full gage and near-full gage portions of the lead mill 12 engage the casing without engaging the whipstock 14. The novel features of this lead mill 12 are its efficient use of space at its nose so as to create concentrated fluid passageways for fluid orifices, such as three nozzle ports 34 and tubular connection hole 32 at the plateau and large recessed debris removal channels 38 to accommodate the flow of debris from those orifices. Key to making efficient use of space is the lead mill shear bolt cavity 23 that is not bored through from one side of the lead mill to the other, nor is it bored into the axial fluid-carrying bore inside the lead mill 12.

An additional novel feature is that the lead mill and follow mill bodies may have their recessed debris removal channels 38 timed to match each other, enabling a more efficient milling and debris removal operation. In other words, the lead mill and follow mill may have the same number of debris removal channels and furthermore these debris channels may be aligned, such that debris channels of the lead and follow mills are the same in number and furthermore the trailing ends of debris channels from the lead mill align with leading ends of debris channels from the follow mill for improved flow of debris and for smoother milling.

In summary, therefore, the present disclosure provides a bi-mill for milling an opening through a wellbore casing in initiating a preplanned lateral drilling path in departure from the wellbore axis and through the wellbore casing or wellbore rock formation, and further continuing into underground rock formation outside the wellbore and in the direction of the preplanned lateral drilling path and as guided by a wellbore whipstock. The bi-mil includes an assembly having a lead mill and a follow mill. The lead mill threadably attaches to the follow mill. The lead mill includes a body forming a structural base for the lead mill, and further includes a smooth bore in the lead mill into which a circumferentially-grooved shear bolt is inserted. A threaded end threadably attaches to the wellbore whipstock and with the unthreaded end inserts into the smooth bore. The lead mill further includes a plurality of fluid passageways wherein the smooth bore has a sufficiently shallow depth so as to not intersect with the internal fluid passageways and so as to not exit the distal side of the lead mill. The lead mill further includes an external bore and a retaining pin inserted through the external bore for intersecting the smooth bore.

The shear bolt includes a groove, wherein the retaining pin further inserts into the groove for retaining the shear bolt within the smooth bore. The lead mill further includes a nose and at least four fluid passageways leading from a central axial fluid passageway inside the lead mill and exiting a plateau defined as the end portion of the nose with an area perpendicular and obtuse in relation a central fluid passageway of the lead mill. A plurality of debris removal channels form recessed paths within the body and a plurality of exit paths for channeling milled debris away from the lead mill and in the direction of the follow mill. A plurality of milling inserts positioned on the lead mill and in rows adjacent the debris removal channels for milling through the wellbore casing or wellbore rock formation and into the underground rock formation. At least three rows of milling inserts extend from the center of the plateau down the side of the lead mill in continuity.

The debris removal channels further form in a helical configuration for guiding debris milled by the inserts along the debris removal channels and away from the lead mill and in the direction of the follow mill. The lead mill and follow mill are configured to follow a path defined by the wellbore whipstock to continue milling through the wellbore casing and into the underground rock formation a distance sufficient to initiate the preplanned drilling path.

The foregoing description of embodiments is provided to enable any person skilled in the art to make and use the subject matter. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the novel principles and subject matter disclosed herein may be applied to other embodiments without the use of the innovative faculty. The claimed subject matter set forth in the claims is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. It is contemplated that additional embodiments are within the spirit and true scope of the disclosed subject matter. 

What is claimed is:
 1. A bi-mill for milling an opening through a wellbore casing in initiating a preplanned lateral drilling path in departure from the wellbore axis and through the wellbore casing or wellbore rock formation, and further continuing into underground rock formation outside the wellbore and in the direction of the preplanned lateral drilling path and as guided by a wellbore whipstock, comprising: an assembly comprising a lead mill and a follow mill, said lead mill threadably attached to said follow mill, said lead mill comprising; a body forming a structural base for said lead mill, and further comprising: a smooth bore in the lead mill for receiving a circumferentially-grooved shear bolt, said shear bolt comprising a threaded end threadably attached to the wellbore whipstock and with the unthreaded end inserted into said smooth bore, said lead mill comprising a plurality of fluid passageways wherein said smooth bore comprises a sufficiently shallow depth so as to not intersect with internal fluid passageways and so as to not exit the distal side of said lead mill, said lead mill further comprising an external bore and a retaining pin inserted through said external bore for intersecting said smooth bore; and said shear bolt comprising a groove, wherein said retaining pin further inserts into said groove for retaining said shear bolt within said smooth bore; said lead mill further comprising a nose and wherein at least four fluid passageways lead from a central axial fluid passageway inside said lead mill and exit at a plateau defined as the end portion of said nose with an area perpendicular and obtuse in relation a central fluid passageway of said lead mill; a plurality of debris removal channels formed as recessed paths within said body and forming a plurality of exit paths for channeling milled debris away from said lead mill and in the direction of said follow mill; a plurality of milling inserts positioned on said lead mill and positioned in rows adjacent said debris removal channels for milling through said wellbore casing or wellbore rock formation and into said underground rock formation, with at least three rows of milling inserts originating contiguously from the center, and extending from the center, of said plateau down the side of said lead mill in continuity; said debris removal channels further formed in a helical configuration for guiding debris milled by said inserts along said debris removal channels and away from said lead mill and in the direction of said follow mill; said lead mill and said follow mill configured to follow a path defined by the wellbore whipstock to continue milling through said wellbore casing and into the underground rock formation a distance sufficient to initiate said preplanned drilling path.
 2. The bi-mill of claim 1, wherein said plurality of milling inserts further comprise cylindrical inserts on both said lead mill and follow mill, said cylindrical inserts comprised of a tungsten carbide with a high percentage (10.5%-12%) of cobalt.
 3. bi-mill of claim 1, wherein said plurality of milling inserts further comprise cylindrical inserts on both said lead mill and follow mill, said cylindrical inserts comprised of a top layer of polycrystalline diamond (“PCD”) on an exposed cutting face.
 4. The bi-mill of claim 1, further comprising cylindrical inserts with a diameter of 13.437 mm composed of a sintered carbide with a polycrystalline diamond (“PCD”) layer on top of the cylinder at its cutting face.
 5. The bi-mill of claim 1, wherein said bypass valve is located uphole in the workstring for receiving sufficient flow and pressure for passing along flow and pressurizing a piston inside a running tool downhole from the bypass valve and in close proximity to said bi-mill.
 6. The bi-mill of claim 1, wherein said bi-mill attachably connects to the wellbore whipstock with a shearable fastener for enabling wellbore whipstock-setting and wellbore departure milling to be completed in a single trip into the wellbore.
 7. The bi-mill of claim 1, wherein said nozzle ports are capped to form capped shearable nozzles preventing fluid flow through said nozzle ports for supplying pressure to a hydraulic tubular connector for hydraulic anchor actuation.
 8. A method for milling an opening through a wellbore casing in initiating a preplanned lateral drilling path in departure from the wellbore axis and through the wellbore casing or wellbore rock formation, and further continuing into underground rock formation outside the wellbore and in the direction of the preplanned lateral drilling path and as guided by a wellbore whipstock, comprising the steps of: providing a bi-mill comprising an assembly, said assembly further comprising a lead mill and a follow mill, said lead mill threadably attached to said follow mill, said method further comprising the steps of; providing a structural base for said lead mill using a body, and further comprising the steps of; providing a smooth bore in the lead mill into which a circumferentially-grooved shear bolt is inserted, with a threaded end threadably attached to the wellbore whipstock and with the unthreaded end inserted into said smooth bore, said lead mill comprising a plurality of fluid passageways wherein said smooth bore comprises a sufficiently shallow depth so as to not intersect with internal fluid passageways and so as to not exit the distal side of said lead mill, said lead mill further comprising an external bore and a retaining pin inserted through said external bore for intersecting said smooth bore; and providing said shear bolt to comprise a groove, and further inserting said retaining pin into said groove for retaining said shear bolt within said smooth bore; providing a nose on said lead mill wherein at least four fluid passageways lead from a central axial fluid passageway inside said lead mill and exit at a plateau defined as the end portion of said nose with an area perpendicular and obtuse in relation a central fluid passageway of said lead mill; providing a plurality of debris removal channels formed as recessed paths within said body and channeling milled debris away from said lead mill and in the direction of said follow mill using a plurality of exit paths associated with said debris removal channels; milling through said wellbore casing or wellbore rock formation and into said underground rock formation using a plurality of milling inserts positioned on said lead mill, said plurality of milling inserts positioned in rows adjacent said debris removal channels for, with at least three rows of milling inserts originating contiguously from the center, and extending from the center, of said plateau down the side of said lead mill in continuity; guiding debris milled by said inserts along a plurality of debris removal channels and away from said lead mill and in the direction of said follow mill using debris removal channels formed in a helical configuration on said lead mill; directing said lead mill and said follow mill to follow a path defined by the wellbore whipstock to continue milling through said wellbore casing and into the underground rock formation a distance sufficient to initiate said preplanned drilling path.
 9. The method of claim 8, further comprising the step of providing said plurality of milling inserts insert to further comprise cylindrical inserts on both said lead mill and follow mill, said cylindrical inserts comprised of a tungsten carbide with a high percentage (10.5%-12%) of cobalt.
 10. The method of claim 8, further comprising the step of providing said plurality of milling inserts to further comprise cylindrical inserts on both said lead mill and follow mill, said cylindrical inserts comprised of a top layer of polycrystalline diamond (“PCD”) on an exposed cutting face.
 11. The method of claim 8, further comprising the step of providing cylindrical inserts with a diameter of 13.437 mm composed of a sintered carbide with a polycrystalline diamond (“PCD”) layer on top of the cylinder at a cutting face
 12. The method of claim 8, further comprising the step of providing said bypass valve located uphole in the workstring for receiving sufficient flow and pressure to actuate for passing along flow and pressurizing a piston inside a running tool downhole from the bypass valve and in close proximity to said bi-mill.
 13. The method of claim 8, further comprising the step of providing said bi-mill for attachably connecting to the wellbore whipstock with a shearable fastener for enabling wellbore whipstock-setting and wellbore departure milling to be completed in a single trip into the wellbore.
 14. The method of claim 8, further comprising the step of providing said nozzle ports as capped to form capped shearable nozzles preventing fluid flow through said nozzle ports for supplying pressure to a hydraulic tubular connector for hydraulic anchor actuation.
 15. A method for manufacturing a bi-mill for milling an opening through a wellbore casing in initiating a preplanned lateral drilling path in departure from the wellbore axis and through the wellbore casing or wellbore rock formation, and further continuing into underground rock formation outside the wellbore and in the direction of the preplanned lateral drilling path and as guided by a wellbore whipstock, comprising the steps of: making an assembly to comprise a lead mill and a follow mill, said lead mill threadably attached to said follow mill, said lead mill comprising; making a body forming a structural base for said lead mill, and further comprising the steps of: making a smooth bore in the lead mill into which a circumferentially-grooved shear bolt is inserted, with a threaded end threadably attached to the wellbore whipstock and with the unthreaded end inserted into said smooth bore, said lead mill comprising a plurality of fluid passageways wherein said smooth bore comprises a sufficiently shallow depth so as to not intersect with internal fluid passageways and so as not to exit the distal side of said lead mill, said lead mill further comprising an external bore and a retaining pin inserted through said external bore for intersecting said smooth bore; and making said shear bolt to comprise a groove, wherein said retaining pin further inserts into said groove for retaining said shear bolt within said smooth bore; making said lead mill further to comprise a nose and wherein at least four fluid passageways lead from a central axial fluid passageway inside said lead mill and exit at a plateau defined as the end portion of said nose with an area perpendicular and obtuse in relation a central fluid passageway of said lead mill; making a plurality of debris removal channels to comprise recessed paths within said body and forming a plurality of exit paths for channeling milled debris away from said lead mill and in the direction of said follow mill; making a plurality of milling inserts positioned on said lead mill and positioned in rows adjacent said debris removal channels for milling through said wellbore casing or wellbore rock formation and into said underground rock formation, with at least three rows of milling inserts extending from the center originating contiguously from the center, and extending from the center, of said plateau down the side of said lead mill in continuity; making said debris removal channels in a helical configuration for guiding debris milled by said inserts along said debris removal channels and away from said lead mill and in the direction of said follow mill; and making said lead mill and said follow mill configured to follow a path defined by the wellbore whipstock to continue milling through said wellbore casing and into the underground rock formation a distance sufficient to initiate said preplanned drilling path.
 16. The bi-mill manufacturing method of claim 15, further comprising the steps of making said plurality of milling inserts to further comprise cylindrical inserts on both said lead mill and follow mill, said cylindrical inserts comprised of a tungsten carbide with a high percentage (10.5% -12%) of cobalt.
 17. The bi-mill manufacturing method of claim 15, further comprising the steps of making said plurality of milling inserts further comprise cylindrical inserts on both said lead mill and follow mill, said cylindrical inserts comprised of a top layer of polycrystalline diamond (“PCD”) on the exposed cutting face.
 18. the bi-mill manufacturing method of claim 15, further comprising the steps of making cylindrical inserts with a diameter of 13.437 mm composed of a sintered carbide with a polycrystalline diamond layer on top of the cylinder at its cutting face
 19. The bi-mill manufacturing method of claim 15, further comprising the steps of making said bypass valve to be located uphole in the workstring for receiving sufficient flow and pressure to actuate for passing along flow and pressurizing a piston inside a running tool downhole from the bypass valve and in close proximity to said bi-mill.
 20. The bi-mill manufacturing method of claim 15, further comprising the steps of making said bi-mill attachably connects to the wellbore whipstock with a shearable fastener for enabling wellbore whipstock-setting and wellbore departure milling to be completed in a single trip into the wellbore. 